Titanium dioxide is used as white pigments in a wide variety of the fields such as paints, inks, plastics and papers, and those which are commercially available at present are roughly classified into anatase type and rutile type according to their crystalline forms. Those of anatase type have the features of higher reflectance for light of short wavelength (showing bluish color tone), smaller absorption of light of ultraviolet portion, lower hardness, lower dielectric constant, and smaller specific gravity as compared with those of rutile type, but are inferior in basic properties of pigments, namely, smaller in refractive index for visible light and lower in opacity than those of rutile type. Thus, the features of anatase type titanium dioxide are not sufficiently utilized. The pigment concentration used in resin compositions which require high opacity is usually not less than 0.5 part by weight of titanium dioxide pigment for 1 part by weight of resin component in the case of paints, inks or the like, and usually not less than 0.05 part by weight in the case of plastics or the like. Commercially available anatase type titanium dioxide pigments have an average particle diameter in the range of 0.1-0.18 μm, and the reason for the anatase type titanium dioxide pigments being low in opacity is that the particle diameter is too small as compared with optimum particle diameter which can give theoretically the highest opacity, although the reason might be due to the basic property that anatase type titanium dioxide pigments are low in refractive index for visible light as mentioned above.
As a method for producing anatase type titanium dioxide pigments having an average particle diameter of not less than 0.2 μm, there is known a method which comprises adding to hydrous titanium oxide a potassium oxide corresponding to 0.2-0.6% by weight calculated as K2O based on the weight of TiO2 in the hydrous titanium oxide and a phosphorus oxide corresponding to 0.15-0.55% by weight calculated as P2O5 as calcination treating agents, preferably further adding an aluminum compound in an amount of not less than 0.2% by weight calculated as Al2O3, and calcining the mixture with heating by gradually raising the temperature from 460° C. to 1020° C. over a certain period of time (JP-A-8-225324). Furthermore, there is known another method which comprises adding to hydrous titanium oxide an aluminum compound corresponding to 0.1-0.5% by weight calculated as Al2O3, a potassium compound corresponding to 0.1-0.7% by weight calculated as K2O and a phosphorus compound corresponding to 0.2-1% by weight calculated as P2O5 based on the weight of TiO2 in the hydrous titanium oxide as calcination treating agents, and calcining the mixture by heating at 1000° C. or higher (JP-A-9-188518).